It is an ideal model to prepare hydrogen-rich gas through steam gasification of solid organics. In order to achieve this, at least two problems need to be solved: providing heat required by steam gasification, and eliminating or decreasing tar in product gas.
Chinese Patent for Invention No. ZL200610113063.3 appears to disclose a decoupling fluidized bed gasification method and device. There, a fluidized bed reactor is divided into two interconnected rooms, in which one room is mainly used for drying and pyrolysis of solid fuel, and the other is used for semicoke-gasification and modification of tar and hydrocarbon. Heat required by pyrolysis and gasification is provided via combustion reaction of raw materials and semicoke with air or oxygen, which are fed into a same reaction space. The patent also provides a dual fluidized bed reaction device and method characterized by using the circulation of solid heat carrier, wherein the heat required by pyrolysis and gasification is partly provided by the combustion of unreacted semicoke in another fluidized bed reactor. Due to the employment of inner combustion for supplying heat, the gasification product gas would comprise inert nitrogen unless employing pure oxygen gasification agent. The limitation of fluidized gasification reactor also lies in: low reaction temperature; short stay time, which causes the conversion of tar and hydrocarbon insufficient; and high dustiness of the product gas. In addition, part of raw materials is directly combusted to supply heat, and thus the hydrogen is mainly converted into water, rather than efficiently enters into hydrogen-rich product gas, which is unreasonable from the view of element utilization.
Austria Vienna University of Technology purportedly developed a biomass gasification process with Fast Internally Circulating Fluidized Bed (FICFB) (reference: http://www.ficfb.at/). The structure of FICFB gasification reactor mainly comprises two reaction spaces: bubbling fluidized bed pyrolysis-gasification zone and fluidized bed rising-combustion zone, and the solid heat carrier circulated within these two zones. The solid heat carrier is heated through combustion of semicoke in the combustion zone and is circulated back to the pyrolysis zone and gasification zone to supply heat required by steam gasification and pyrolysis of biomass in the pyrolysis zone and the gasification zone. Then the solid heat carrier is re-fed into the combustion zone to start the next cycle. The gases of the two zones are separated with each other, therefore, hydrogen-rich gas without nitrogen can be produced. Pyrolysis and gasification of FICFB technology are performed at a same reaction space, which is hard to achieve independent control over pyrolysis and gasification, and has limitation to the adaptability of different raw materials. Both the stay time of biomass pyrolysis volatile matter in fluidized bed gasification reactor and the contacting time of the volatile matter with solid heat carrier are short, which leads to insufficient conversion of tar and high tar content of product gas. Therefore, the improvement of gasification efficiency is restrained. Where biomass, young brown coal, etc. are used as raw materials, the generated gaseous product may have a large amount of dust due to the pulverization of the raw materials during pyrolysis gasification process. If the dust cannot be efficiently removed in hot condition, the dust and the tar in gaseous product may form viscous mixture in the following condensation-purification process, which affects normal operation of system.
Chinese Patent for Invention No. ZL200710011214.9 appears to provide a method that enables independent control over pyrolysis of solid fuel raw materials, further decomposition and conversion of tar and hydrocarbon in the gaseous product generated by pyrolysis, and supplying heat to the reactions by combusting the semicoke from pyrolysis. The method is achieved through the circulation of solid heat carrier within three tandem reactors, which are moving bed pyrolysis reactor, moving bed gasification reactor, and riser and combustion reactor. The reactions respectively performed within the three reactors are: pyrolysis of solid fuel raw materials, steam gasification of gaseous product (including tar and low-carbon hydrocarbon) generated by pyrolysis, and combustion of semicoke and re-heating and rising of solid heat carrier. The limitation of the method is that, since the pyrolysis reactor and gasification reactor are tandem connected, the solid heat carrier from the riser and combustion reactor passes through the pyrolysis reactor and gasification reactor in turn, and then loops back to the riser and combustion reactor; therefore, the running conditions of the pyrolysis reactor and gasification reactor restrict each other. The temperature of the solid heat carrier fed into the pyrolysis reactor fully depends on the reaction degree within the gasification reactor, and the kinds and quantity of solid heat carrier fed into the pyrolysis reactor and gasification reactor cannot be respectively and independently controlled either. Therefore, it may be hard to achieve the goal that both pyrolysis reactor and gasification reactor are running at their respective optimal running conditions.